Mastigias are photosynthetic but they don't seem to need a huge amount of light to grow. A small LED designed for a planted tank seems to be fine and they can even go periods (still experimenting with this) with no light as long as they are well fed.

If you are planning on keeping them in a Pulse 80 you should remove the lid and ad some extra light. As long as you do this they should be fine. We have actually found they grow very quickly!

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